A conventional tire inflation pressure monitoring system for a vehicle is known in which items of information from a tire inflation pressure sensor for monitoring a tire inflation pressure are wirelessly transmitted from a transmitting portion provided at a tire or a wheel to a receiving portion provided at a vehicle side. Generally, weak electromagnetic wave is utilized in such a tire inflation pressure monitoring system because such electromagnetic wave can be utilized without a license application. Accordingly, a highly sensitive receiving antenna would be required. Then, for ensuring sufficient level of received signals, a larger antenna needs to be provided. For avoiding use of such a larger antenna, a signal wire of a wheel speed sensor is utilized as a receiving antenna according to techniques described in JPH10-309914A (document 1). As a result, a receiving antenna for receiving signals from the tire inflation pressure sensor is not in need of being provided near a wheel. Accordingly, wiring can be abolished from the receiving antenna to a control apparatus such as an electronic control unit (ECU).
However, in the techniques according to document 1, because the signal wire of the wheel speed sensor is utilized as the receiving antenna of the tire inflation pressure monitoring system, and the signal wire is generally not shielded, a signal to noise ratio (S/N ratio) of received signals tends to be decreased when the signal wire is utilized as the receiving antenna. As a result, receive of necessary signals of the tire inflation pressure sensor would be difficult. If, for compensating this, the signal wire is shielded in order for improving the S/N ratio, entire of long signal wire needs to be shielded from near the wheel to the control apparatus provided in an engine room or a cabin. Such configuration would cause increase of costs.
A need thus exists for a tire inflation pressure detection apparatus which can stably receive signals of a tire inflation pressure, and which is low in cost. The present invention has been made in view of the above circumstances and provides such a tire inflation pressure detection apparatus.